FIG. 1 shows the structure of conventional reed relay 100. Signal conductor terminals, or signal line terminals, 117a and 117b at both ends of reed switch 101 are held near both ends of electrostatic shielded pipe 103 by insulators 102a and 102b, which are plates called bushings and have holes in the center through which the terminals pass. This electrostatic shielded pipe 103 is inserted into the cylindrical hollow part of coil bobbin 104. Coil 105 for excitation is wound and installed in the concave part around the outside of coil bobbin 104. This concave part is further packed with resin 106 and this is covered by magnetic shield case 107. Here, electrostatic shielded pipe 103 and coil bobbin 104 are adjacent and contact one another. Moreover, when reed relay 100 is used in the circuit, it is preferred that electrostatic shielded pipe 103 is connected to guard wires that and function as an active guard or passive guard. Please note that unless otherwise specified, the same symbols are used to describe the same structural elements in the figures of the present Specification.
Nevertheless, reed relay 100 poses many problems when used for the measurement of very small currents on the order of fA in terms of the relationship between the latency time or wait time until the measurement values stabilize and offset current reduces, as described below:
FIG. 2 shows the measurement results of the offset current of reed relay 100. The x-axis shows the time that has passed when 0 seconds serves as the time when excitation current begins to flow to the coil, and the y-axis shows the current flowing through the signal conductors of the relay as detected by an ammeter for very low currents with a guard feature. In this case, the guard terminals of the ammeter are wired so that they are connected to the electrostatic shielded pipe of the relay, with the voltage of the signal conductors being constant at 0V, and a 10 mA rated current flows as excitation current to the coil. Please note the fact that the reed switch is turned on with excitation of the coil in this case. According to FIG. 2, excitation current begins to flow to the coil and the negative polarity current gradually increases in approximately 80 seconds to peak at -6 fA. Thereafter the negative polarity current settles down with convergence to approximately 0 fA approximately 300 seconds after starting excitation and stabilization to the steady state.
Thus, conventional reed relays are inappropriate for high-speed and high-precision measurement of very small currents, because an offset current of approximately several fA flows for approximately 100 seconds beginning immediately after the relay has been turned on.
Leakage current or thermo-electromotive force due to a contact potential difference between different types of metals, as described in Japanese Patent Laid-open (Kokai) No. Hei 2(1990)-68,829, and dielectric absorption in an insulator, as described in Japanese Patent Laid-open No. Hei 8(1996)-279,314 were considered to be factors of the above-mentioned offset current in the past, but of course unclear points remain. That is, the explanation of leakage current being transmitted over the surface of an insulator or of current being generated by potential difference between relay terminals due to thermo-electromotive force, which is caused by the difference in the amount of heat conduction towards the both ends of relay terminals, applies only to the steady state current and contradicts the phenomenon of offset current naturally converging at 0 fA. Moreover, the voltage of the signal conductors was constant at 0 V in the measurements shown in FIG. 2, and therefore, it does not appear that a potential difference with which dielectric absorption would occur was produced between the signal conductors and the electrostatic shielded pipe.
In any case, in the past very low currents on the order of fA were observed after waiting for approximately 100 seconds until the offset current stabilized, and therefore, high-speed measurement of the very low current was not possible. Very low currents could also be measured by reducing wait time, recognizing that the results would be inaccurate. Nevertheless, the measurement devices have become faster each year and therefore, it is necessary to develop a high-performance reed relay for very low currents with which the wait time can be curtailed and speed can be increased.
The present inventor hypothesized the following based on the fact that the above-mentioned offset current is due to thermally stimulated current that is produced when Joule effect heat generated by the coil propagates or transmits to the contact surface between the metal and the insulator:
That is, by means of the reed relay in FIG. 1, heat that has been generated by the coil is transmitted as shown below.
One is coil (105).fwdarw.coil bobbin (104).fwdarw.electrostatic shielded pipe (103).fwdarw.bushing (102a, 102b), and the other is coil (105).fwdarw.resin (106).fwdarw.magnetic shield case (107).fwdarw.air.
Plastics and resin materials with high insulating performance generally have thermal conductivity that is two to three orders of magnitude lower than metals, and therefore, heat is conducted via the above-mentioned two routes on the order of materials with EQU good.fwdarw.poor.fwdarw.good.fwdarw.poor
conductivity. As a result, thermal resistance becomes several 10 K/W and, for instance, when as much as 0.1 W heat is generated by the coil, the temperature of the electrostatic shielded pipe will probably also rise by several K.
The electrons trapped at the surface on the electrostatic shielded pipe 103 side of bushings 102a and 102b are excited by thermal energy with this rise in temperature and are released to inside electrostatic shielded pipe 103. In this case, electrons are probably fed from the sides of signal conductor terminals 117a and 117b to bushings 102a and 102b, which are insulators, in a form that maintains electric neutrality.
The fact that this hypothesis is correct was demonstrated as follows by experiments. First, as shown in FIG. 3, heat-absorbing element 311, which is called a Peltier device, is fastened directly above coil bobbin 104 of the relay. Coil 105 is not excited. The voltage of the signal conductors is constant at 0 V and only Peltier device 311 is operated. That is, heat passes through coil 105 and is transmitted in the opposite direction from the case for the measurements in above-mentioned FIG. 2 in the sequence such as: EQU Peltier element (311).rarw.magnetic shield case (107).rarw.resin (106).rarw.coil (105).rarw.coil bobbin (104).rarw.electrostatic shielded pipe (103).rarw.bushings (102a, 102b), and heat is absorbed or cooling takes place.
The offset current waveform in this case is the waveform shown in FIG. 4A. The inventors discovered that in this case, the polarity of the offset current is positive and opposite from FIG. 2.
FIG. 4B shows the results of further flowing a rated current of 10 mA to coil 105 and performing the same operation as with Peltier device 311 under the above-mentioned conditions based on the above-mentioned discovery. It is clear that the current of negative polarity in FIG. 2 and the current of positive polarity in FIG. 4A cancel each other out so that the offset current is controlled.
Based on the above-mentioned results, the mechanism of the above-mentioned offset current appears to be as follows:
The heat generated from the coil is transmitted to the electrostatic shielded pipe via the coil bobbin and reaches the bushings. Electrons are trapped at the surface energy level (Fermi level) on the bushing side of the surface of contact between the bushings, which are insulators, and the electrostatic shield. These electrons are excited when exposed to thermal energy and jump over the energy barrier and are released into the metal as free electrons (for instance, refer to FIG. 6.9 on page 36 of Y. Murata, "Static electricity between surfaces and polymers," 1988, Kyoritsu Shuppan). Electrons are newly fed from the signal conductor side in order to maintain the electric neutrality of the bushing after electrons have flown out at the surface level on the signal conductor side, which is the side opposite to the shielded pipes, in this case. This apparently becomes the negative polarity current on the order of femtoamperes (that is, thermally stimulated current) that is observed.
The mechanism by which thermally stimulated current is generated in high-insulating materials consisting of fluorinated polymer resin, such as PTFE (polytetrafluoroethylene) and FEP (fluorinated ethylene propylene copolymer), etc., is described in detail in R. L. Remke, H. Von Seggem, "Modeling of thermally stimulated currents in polytetrafluoroethylene (PTFE)," J. Appl. Phys. 54(9), pp 5262 to 5266, September, 1983.
In contrast to the above-mentioned, electrons in the electrostatic shielded pipe near the bushings lose their energy and can be trapped on the trap level inside the bushings during the process whereby the Peltier device operates to take thermal energy from the bushing. Electrons at the surface on the side of the signal conductor terminal of the bushing are released to the signal conductor to maintain electrical neutrality and a positive polarity current is observed.
Thus, it can be theoretically supported that offset current is generated due to the Joule heat effect of the coil.
When conventional technology for reed relays is re-examined based on the results of the above-mentioned discussion, it is clear that conventional reed relays are not an effective countermeasure for handling thermally stimulated currents.
For instance, by means of the technology disclosed in Utility Model Laid-open No. Hei 5(1993)-31,078, the coil bobbin and electrostatic shield come into contact over their entire surface and the same mechanism as discussed in FIG. 1 can be used. Thus, an insulator is packed in the electrostatic shielded pipe up to the reed switch. Therefore, the structure is one with which a thermally stimulated current can easily flow.
By means of a different technology, a structure is used wherein the reed relay 100 in FIG. 1 has a large hollow cylinder of coil bobbin 104 into which electrostatic shielded pipe 103 will be inserted, and when coil bobbin 104 and electrostatic shielded pipe 103 are soldered to the circuit board, they are soldered so that they are supported by the lead wires of coil 105 (not illustrated) and the signal conductor terminals 117a and 117b of the reed switch at a position where the two will not make direct contact. It appears that as a result, there is only air between the coil bobbin and the electrostatic shielded pipe and therefore, heat will very rarely be transmitted. Nevertheless, when assembled, first the coil bobbin is soldered to the board, then the electrostatic shielded pipe is inserted, and both signal terminals of the reed switch are processed by cutting and bending to adjust the position to the two ends of the electrostatic shielded pipe. Finally, it is necessary to carefully solder the electrostatic shielded pipe to the board so that there is a specific distance maintained from the inside walls of the hollow part of the coil bobbin. Thus, assembly is difficult and production cost is too high by this technology.
Yet another technology is disclosed in Japanese Patent Laid-open No. Hei 2(1990)-68,829 that was previously presented. By means of the technology disclosed in this patent, two excitation coils are set up. The both of coils are always excited regardless whether the relay turns on or off so that the total amount of heat generated by the coils is kept constant. Opening and closing of the contacts is performed by changing the combination of the directions of the current flowing to the coils. By means of this technology, the temperature of the relay is constant and therefore, changes in thermo-electromotive force due to changes in the temperature of the reed switch can be minimized. Moreover, it appears that this technology is also effective in suppressing thermally stimulated current. However, excitation currents must always be flowing to the coils by this method, and therefore, as the number of relays is increased, the power consumption is also increased. A large-capacity power source and large cooling facility become necessary, which is not economical. Moreover, this technology is inconvenient because very low currents cannot be measured during the 100 seconds when coil excitation begins and the steady state is reached.
As previously explained, these problems that were first made clear through the discussions of the inventor are attributed to (1) the fact that the structure is one with which the heat generated by the coil is readily transmitted to the electrostatic shielded pipe, and (2) further, the structure is one with which the heat generated by the coil is readily transmitted from the electrostatic shield to insulators that hold the signal conductors of the reed switch.
Consequently, the object of the present invention is to solve the above-mentioned problems and present a reed relay with which offset current is controlled or suppressed competently, even if the coil is excited for relay operation, high-speed and high-accuracy measurement of very low signals is possible, and further, power consumption is low and production costs during assembly are low.